Post-fab visualization: the deceptiveness of SEM pictures
In a Scanning Electron Microscope, the depth of field (DOF) is inversely proportional to the both the aperture size and magnification. Although this is in some ways similar to an Optical Microscope, the DOF is much higher for an SEM for the same magnification. For a 100 micron objective lens with a 10x eyepiece (total mag of 1000x), the DOF is around 1 micron. For an SEM at under 1000x mag, the DOF is around 40 microns (with a 100 micron aperture, 10 mm Working Distance).
Lets say you have a part machined by Acme Laser, and they send you SEMs of the part. Testing reveals that something is just plain wrong with the part, but the SEMs look great. When you check with an optical microscope, things start looking bad- the part has been made unevenly. The reason the SEM led you astray is that the part looks focused over a greater depth and hence conveys a mental image of good micromachining. When you put it under an Optical Microscope with a similar magnification, the depth of field is much smaller, and you will have to keep moving the focus over a greater distance to look at all the features. In essence, unless you are a computer and can store the image details at every 1 micron depth of focus, the part looks uneven- which conveys a mental picture of poor micromachining.
So which should you use? An experienced micromachinist can make do with an Optical Microscope, but for showing the same part to potential investors or to spice up publications, you are much better off using an SEM, even though the SEM provides black and white pictures (while the OM provides color).
There is a caveat, though: Often, a well micromachined part may not be what is really needed. You might actually end up needing a softer focused part to avoid sharp edges and for better fit- as in biomedical applications. Read about it in another article that will appear soon.
Another thing to remember: An SEM gives much higher resolution than an Optical Microscope primarily because the wavelength of an electron beam is much smaller- less than a nanometer compared to about 500 nm in the visible spectrum. But then, a laser can not micromachine features smaller than around 2 microns (usual case scenario for a Flourine laser at 157 nm). So what’s the point using a metrology/visualization tool with a much higher resolution- unless you are loking at post-fab debris and compostional changes. That’s another topic altogether.
One can get down to 5 micron features or smaller when using 157nm of Fluorine Excimer Laser for micromaching. White light interferometers (a mirau on a induatrial micrscope) kind of gives the best of both SEM and optical microscopy. One gets an extended depth of focus due to axial scanning and since the Z can be encoded 3D reconstruction is possible.
I am not sure if you had a question, and so have been too lazy to comment on this. Apologies.
The theoretical resolution of a 157nm is much smaller than 5 microns. In fact, we routinely achieve this using 193nm lasers. For example, in the area of diamond marking, where the character sizes are as small as 50 microns, the spot size is often smaller than 2 microns. Of course, there is a difference between spot size and feature size…
White light interferometry does NOT give you much horizontal resolution, that is, resolution as understood in the world of optical microscopy. Resolution here is determined by the RST’s objective’s NA and the wavelength of light (in this case an average for white light), just like it is in traditional optical microscopy.
The resolution for a White light interferometer comes from the very high resolution of piezo material while scanning vertically, and the very short depth within which interference patterns appear and disappear when using white light.
The RSTs I worked with used 2 types of measurement methods: VSI (vertical scanning interferometry) and PSI (phase shifting interferomtry). VSI has a much higher reolution, and uses fringe modulation information while scanning vertically. PSI uses fringe intensity information while making just a few steps of vertical motion.
In my several years of working with SEMs and RSTs as micrometrology tools for laser micromachining, I have found RSTs to provide only superficial information that is fit only for publications with fancy bar graphs and surface plots. Sorry for the cynicism.
SEMs on the other hand don’t do much to help process engineers in their day to day laser micromachining work. They are, again, good for publications. Also excellent for investor relations
Nothing beats a good old Optical Microscopy setup with good lighting, good reticles, and extra long working distance
PS: While at it, can I also rubbish the misleading claims of RST makers of providing Angstrom level resolution…
Edit: I re-read the article and realized I had written 200 microns instead of 2 microns. Now I understand what you meant…Sorry!